The present invention relates to a holding structure for an optical element such as a mirror or a lens.
Japanese Patent Application Laid-open Publication 2000-231077 discloses a holding structure for an optical element (integrator lens). In the holding structure, the optical element is elastically biased by a biasing member such as a plate spring so as to be pressed against positioning means provided in a unit case, thereby positioning of the optical element is achieved.
However, the holding structure has low reliability against external impacts and the like. The biasing force by the biasing member is preferably as weak as possible to prevent distortion of the optical element. However, if a large load due to an impact caused by falling or the like acts on the biasing member in a direction opposite to the direction in which the optical member is elastically biased, the weak biasing force may cause deformation of the biasing member exceeding elastic range and reaching plastic range. Once the deformation reaching the plastic range, the biasing member does not return completely to its original shape and residual distortion remains even when the load is ceased. As a result, the biasing force on the optical element positioning means becomes insufficient to appropriately hold the optical element. Especially, remarkable deformation of the biasing member may severely change posture of the optical element.